In general, the present invention relates to piezoelectric ceramic compositions, articles formed from these compositions, and methods for preparing the piezoelectric ceramic compositions and articles.
Piezoelectric elements are widely used in a variety of electronic components including ceramic resonators, ceramic filters, piezoelectric displacement elements, buzzers, transducers, ultrasonic receivers and ultrasonic generators, etc. Consequently, the use of the piezoelectric ceramic compositions to form the elements is increasing. The different uses or applications require different electromechanical characteristics from the piezoelectric ceramics. Furthermore, there is continued drive towards increasingly smaller electronic components. Consequently, there is a concomitant demand for smaller piezoelectric elements to use in these electronic components. However, many of the smaller electronic components require that the piezoelectric elements provide the same or even greater output power, despite their reduced size.
Existing high power piezoelectric ceramics often do not exhibit suitable electromechanical properties for use in miniaturized electronic devices, such as miniaturized ultrasonic devices. In the current state of the art, the existing piezoelectric elements that are sufficiently small to be used in the miniaturized devices exhibit low capacitance and high electrical impedance. This is inadequate to drive the miniaturized devices. Furthermore, the dielectric loss factor (tan δ) of the current piezoelectric elements is too high resulting in internal heating and dissipative loss, which will significantly decrease the efficiency and output of the device. Consequently, existing piezoelectric ceramics have not provided adequate electromechanical properties for these miniaturized electronic devices.
The electromechanical properties of the piezoelectric ceramics can be altered by varying the specific ceramic composition, the ceramic or molecular structure, and/or the methods and parameters for fabricating the piezoelectric ceramic.
Common piezoelectric ceramics can be formed from of a variety of general classes or types of ceramics. One class is a lead-zirconium titanate ceramic (PZT); another class is a lead-magnesium niobium ceramic (PMN). In many cases, solid solutions of either the PZT or the PMN ceramic are prepared in which discreet components or ceramic particles are distributed either homogeneously or inhomogeneously in the bulk matrix—either the PZT ceramic or the PMN ceramic. The discreet components can be found in the interstitial spaces of the crystal units of the bulk matrix. The added components can provide additional characteristics to the resulting piezoelectric ceramic. Additionally, dopants have been added to the piezoelectric ceramic matrix to modify a variety of factors, including the Curie temperature, the mechanical quality factor, the dielectric dissipation factor, and the like.
In addition, methods used to fabricate the piezoelectric ceramics vary widely. In particular the specific heat treatment regimes and poling processes can have a dramatic effect of on both the physical properties and the electrical properties of the resulting ceramics. It is often problematic to fabricate a suitable ceramic that forms a uniform, single phase having a desired crystal structure and/or that does not crack under the strain in use.
As with many ceramic fabrication techniques, the processing parameters typically are tailored to the specific ceramic composition and its intended use. Furthermore, even the known processing parameters used for a specific class or type of ceramic still can be varied to provide a unique piezoelectric ceramic having desired electromechanical properties. However, the effect of the changes on the electromechanical properties typically cannot be predicted a priori-a without extensive experimentation both as to the composition and to the method of fabrication. Consequently, it is still very difficult to adequately prepare piezoelectric ceramics having the desired electromechanical properties for miniaturized electronic devices.
In light of the above problems, there is a continuing need for advances in the relevant field including new piezoelectric ceramic compositions, piezoelectric elements formed from the compositions, and methods of fabricating the compositions and the elements. The present invention addresses that need and provides a wide variety of benefits and advantages.